Various redox processes can be used to split gas-phase reactants. For example, hercynite, ceria, and M-ferrite redox cycles may be used to split water (vapors) and CO2. The hercynite cycle can be represented by the following equations.
Hercynite cycle:Reduction: CoFe2O4+3Al2O3→CoAl2O4+2Fe2AlO4+½O2  (1)Oxidation: CoAl2O4+2Fe2AlO4+H2O (or CO2)→CoFe2O4+3Al2O3+H2 (or CO)  (2)Overall: H2O→H2+½O2 or CO2→CO+½O2  (3)
Non-stoichiometric ceria cycle can be represented by the following equations.Reduction: 1/δCeO2+Heat→1/δCeO2-δ+½O2(g)  (4)Oxidation: 1/δCeO2-δ+H2O or CO2(g)→1/δCeO2+H2 or CO(g)  (5)
Standard M-ferrite cycle (M=Co, Ni, Mn, Zn, etc.) can be represented by the following equations.Reduction: MxFe3-xO4→[xMO+(3−x)FeO]+½O2  (6)Oxidation: [xMO+(3−x)FeO]+H2O or CO2→MxFe3-xO4+H2 or CO  (7)
The hercynite cycle reduction step (1) can be carried out at temperatures that are substantially lower than those of competing cycles, such as the reduction step (4) of the non-stoichiometric ceria cycle and the reduction step (6) of the standard M-Ferrite Cycle. The hercynite cycle active materials start to undergo thermal reduction at temperatures as low as 940° C. (although reduction is typically performed at 1200 to 1400° C.) [1], while the ceria cycle typically requires a reduction temperature of 1500° C. [2, 3]. The standard ferrite material, on the other hand, starts to undergo thermal reduction at 1190° C.; however, thermal reduction is typically performed at 1450° C. [4].
These systems are normally run as reduction/oxidation (redox) cycles with reduction temperatures typically hundreds of degrees Celsius above the corresponding oxidation temperatures. For example, ceria is typically reduced at 1500° C. and then oxidized at 850° C. to 1000° C. [2, 3]. Likewise, standard ferrite is reduced at 1450° C. and then oxidized between 900 and 1100° C. [4]. This difference in temperature requires a cool-down followed by a heat-up step, both of which take time and inherently result in sensible heat losses and reduced efficiency of the process.
Running the oxidation and reduction steps of the redox cycles at substantially different temperatures may be undesirable for several reasons. For example, systems running such cycle exhibit heat loss with each temperature change. In addition, altering the temperature of the system to run each cycle requires additional time to heat or cool the system that reduces the number of cycles that can be performed during a given period. And, the temperature cycling can cause materials used in the reaction to degrade more quickly than if the reaction could be run at substantially isothermal conditions. Accordingly, improved methods and apparatus for splitting gas-phase reactants using substantially isothermal or isothermal conditions are desired.
Additionally, systems and methods for splitting gas-phase reactants using solar thermal reactors are desired.